In recent years, with electric power shortage becoming serious, there have been challenges that are global rapid adoption of natural energy such as the adoption of wind power generation and solar photovoltaic power generation and the stabilization of power systems (for example, maintaining of frequency and voltage). One technique for addressing the challenges has been attracting attention and this technique is to install high-capacity storage batteries to achieve, for example, smoothing of output variations, storage of surplus power, and load leveling.
One of such high-capacity storage batteries is a redox flow battery (hereafter, sometimes referred to as an RF battery). The RF battery has the following features, for example: (i) a high capacity in the megawatt class (MW class) is easily achieved, (ii) the longevity is long, (iii) the state of charge (SOC) of the battery can be accurately monitored, and (iv) battery output and battery capacity can be independently designed, so that a high degree of freedom in design is provided. Thus, the RF battery is expected to be optimum as a storage battery used for the stabilization of power systems.
The RF battery mainly includes a battery cell including a positive electrode to which a positive electrode electrolyte is supplied, a negative electrode to which a negative electrode electrolyte is supplied, and a membrane disposed between the two electrodes. Typically, an RF battery system is constructed so as to include an RF battery and a circulation mechanism configured to circulate and supply, to the RF battery, electrolytes for the two electrodes. In general, the circulation mechanism includes a positive electrode tank storing a positive electrode electrolyte, a negative electrode tank storing a negative electrode electrolyte, and ducts individually connecting the tanks for the electrodes and the RF battery.
The electrolytes for the electrodes are typically solutions containing, as active materials, metal ions that undergo changes in valence by oxidation-reduction. Typically, there are an Fe—Cr-based RF battery employing iron (Fe) ions as the positive electrode active material, and chromium (Cr) ions as the negative electrode active material, and a V-based RF battery employing vanadium (V) ions as active materials for the two electrodes (refer to Patent Literature 1).
Patent Literature 1 discloses a Mn—Ti-based RF battery employing manganese (Mn) ions as a positive electrode active material, and employing, for example, titanium (Ti) ions as a negative electrode active material. The Mn—Ti-based RF battery is advantageous in that it provides a higher electromotive force than the existing V-based RF battery, and the raw material for the positive electrode active material is relatively inexpensive. In addition, Patent Literature 1 discloses that a positive electrode electrolyte containing manganese ions and also titanium ions enables suppression of generation of manganese oxide (MnO2), so that the Mn2+/Mn3+ reaction proceeds with stability.